It is already known to react liquid maleic anhydride with each of the three dienes referred to above. These reactions take place with almost a quantitative yield and result respectively in endomethylene-tetra-hydrophthalic anhydride, the anhydride of bicyclo, (2,2,1) heptene-5 dicarboxylic-2,3 acid from cyclo-pentadiene, in methyl-4-tetra-hydrophthalic anhydride from isoprene and in methyl-3-tetra-hydrophthalic anhydride from piperylene-trans.
More recently, mixtures have been obtained of methyltetra-hydrophthalic anhydrides from mixtures previously freed from cyclo-pentadiene, or at least impoverished in this hydrocarbon. It is in fact difficult to react quantitatively in the same operation, dienes of such different reactivities as cyclopentadiene, isoprene and piperylene. Furthermore, in the methods of the prior art, when the starting mixtures contained cis piperylene as well as certain ethylene hydrocarbons, the product obtained was contaminated with polymerization products of these hydrocarbons or with co-polymerization products of these hydrocarbons with maleic anhydride, which makes it necessary to work in an inert atmosphere in the presence of a free-radical inhibitor and to purify the product obtained.